The exercise in this video is done seated on a chair, which is good if you have difficulty getting onto the floor. Pelvic positioning is outlined in detail in this video – please pay close attention to the cues for positioning your pelvis.
Because so many people spend a significant amount of time sitting (computers, desks, couches, and cars are the main culprits), piriformis syndrome is on the increase. Read about this on the ST Piriformis Syndrome Wiki. The insidious aspect of this common problem is that it can (in its most severe form) mimic all the neurological deficits caused by lumbar disc disease; disc herniation, for example. Piriformis syndrome is the major cause of sciatica, through the same mechanisms. In any kind of low back pain, its possible contribution needs to be ruled out; these exercises are the place to start this process. As well, and more generally, tight hamstrings are often the result of tight piriformis (and piriformis can cause tight gastrocnemius, too).